1. Field of the Invention
The present invention relates to a high-gain amplifier with low noise and low power dissipation, using field effect transistors.
2. Prior Art and other Conciderations
Field effect transistors (FET) have in the saturation region their maximum output impedance. The output impedance can be low in those transistors wherein, for reasons of construction geometry, it is not possible to saturate the channel region completely. On the other hand, in transistors wherein the dominant noise is the low frequency one (called 1/f noise) the optimum biasing point for the noise is in the linear region of the transistor, where the output impedance is always low and the drain/source voltage is in turn limited.
In the design of high-gain amplifiers, in particular but not exclusively in the dominant-pole ones, wherein a field effect transistor is supplied by a current generator and controlled by the input signal while the output is taken in an intermediate node between the transistor and the current generator, the voltage gain is determined by the product of the transistor transconductance by the impedance seen in said intermediate node, determined by the parallel combination of the output impedances of the transistor and of the current generator.
If the amplifier must also be of the low 1/f noise type, in view of what has been said above it is necessary that the transistor be biased in the linear region, where on the other hand the transistor has a low output impedance. The impedance seen in the output node of the amplifier thus has a modest value and as a consequence the voltage gain becomes low.
Another problem of amplifiers is represented by the quantity of power dissipated, which obviously translates into heat which can be particularly undesirable for uses such as those in low temperature operations.
In view of this state of the art, the essential object of the present invention has been to realize an amplifier with field effect transistors, which at the same time has low-noise and high-gain characteristics.
Another object of the present invention has been that of realizing a high-gain and low-noise amplifier, which also has low power-dissipation characteristics.